Site Characterization and Modeling Considerations for Slopes Involving Fine Grained Strain-softening Soils
Type of DegreePhD Dissertation
Civil and Environmental Engineering
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The potential for strength loss in fine grained soils poses a significant hazard for many geotechnical projects. Damage to infrastructure attributed to strain-softening of clayey soils has been documented in many case histories involving both static and cyclic loading. For critical infrastructure projects, potential deformations due to cyclic loading are increasingly being analyzed using numerical modeling approaches, such as nonlinear deformation analyses (NDAs) for dynamic problems or strength reduction analyses for static problems. These analyses rely on material models which can represent the aspects of soil behavior important to the problem being analyzed. For any analysis method, accurate characterization of a landslide site is key to building reliable models. Supplementing traditional geotechnical explorations, such as borings, with geophysical methods can provide continuous site profiles and help identify changes in soil type or saturation across the site. This dissertation explores the analysis of static and dynamic failures in strain-softening clays through examination of two landslides, the Fourth Avenue landslide in Anchorage, Alaska and a currently moving landslide along Interstate 65 (I-65) in Conecuh County, Alabama. Methods for characterizing and analyzing strain-softening in clays are reviewed, followed by an analysis of the Fourth Avenue landslide using the constitutive model PM4Silt, which can capture cyclic softening of clays at the field scale. Guidance is provided regarding calibration of PM4Silt and the sensitivity of the solution to the input parameters is explored. The results using PM4Silt are then compared to a total stress-based constitutive model to understand how differences in constitutive model complexity affect the results at both the element scale and field scale. Mesh dependency of the solution using both models is explored with and without regularization approaches. The evaluation of strain-softening clays under static loading conditions is explored for the I-65 landslide using both tradition limit equilibrium and more advanced strength reduction analyses. Characterization of the materials at the site is performed using results from borings, electrical resistivity imaging and seismic data, along with laboratory testing. The results from the site characterization are used to build a site model for use in slope stability analyses. Limit equilibrium analyses are performed and the results using circular and noncircular failure surfaces are examined. The limit equilibrium results are also compared to strength reduction analyses using the Modified Hoek-Brown constitutive model to represent nonlinear strength envelopes. Sensitivity of the slope stability analysis solutions to model input parameters are studied. The results from these analyses are then used to determine a probable failure mechanism for the site and to provide recommendations for future analyses of similar landslides. This study shows that simulations using PM4Silt can reproduce the deformation patterns and magnitudes observed at the Fourth Avenue Landslide. Effective stress-based constitutive models are found to be more appropriate than total stress-based models when element level response or a more complete estimation of deformations patterns, both inside and outside the failure mass, is required. Total stress-based models are found to be adequate if the user is primarily interested in whether failure will occur. Mesh dependency is observed in the PM4Silt model, but these effects are reduced by using a displacement-based calibration procedure. Electrical resistivity and seismic methods are shown to be reliable tools for estimating site stratigraphy and can provide information regarding the failure mass and slide plane. The Modified Hoek-Brown constitutive model is shown to be a practical option for representing nonlinear strength envelopes of soils in strength reduction method analyses as slope stability analysis results compare well with limit equilibrium analyses uses power curve fits.